This invention relates generally to optical interferometry. In particular, the invention relates to methods, devices and device components employing interferometers and optical interference filters for processing optical signals. Optical interleavers having parallel phase control elements are described, which are particularly useful for wavelength division multiplexing and demultiplexing applications.
Optical telecommunication systems are capable of efficient and accurate signaling at extremely high rates ranging several mega-bits per second to several tens of giga-bits per second. In addition, optical signaling techniques have significant advantages over non-optical communication methods, such as coaxial cable, copper wire and microwave transmission techniques, which include lower propagation loss, higher channeling capacity and insusceptibility to electromagnetic interference. As a result of these benefits, optical communication systems are prevalent in nearly all existing telecommunication networks and a great deal of research has been directed at developing purely optical telecommunications systems.
As worldwide telecommunications usage continues to expand, the need for greater data-carrying capacity has made potential gains in channeling capacity via optical telecommunications methods especially attractive. To provide additional data-carrying capacity without requiring new optical fiber transmission lines, coarse wavelength division multiplexing and dense wavelength division multiplexing techniques have developed over the last decade. Wavelength division multiplexing is used to increase the transmission capacity of fiber optic communication systems by allowing multiple wavelengths to be transmitted and received over a single optical fiber. In wavelength division multiplexing, a plurality of optical signals of different wavelength are multiplexed by coupling each signal to a common transmission line. The multiplexed transmission signal is then propagated over a single optical medium to a variety of receivers. When received, the multiplexed transmission signal is demultiplexed into discrete channels corresponding to individual wavelengths and detected by a receiver. Typically, signal demultiplexing is achieved by a variety of wavelength selective optical filtering devices including optical interference filters, birefringent filters, cutoff filters, prisms, diffraction gratings and fiber optic devices. Although wavelength division multiplexing provides a simple, effective and inexpensive way of increasing transmission capacity, the number of channels employable over a given wavelength domain is limited by cross talk between transmission channels. Cross talk refers to incomplete separation of selected and non-selected optical channels such that light corresponding to one or more non-selected optical channels remains in combination with a selected channel and is detected. As understood by those skilled in the art, cross talk degrades the overall efficiency and accuracy of an optical communication system and substantially limits the narrowest channel spacing achievable. Accordingly, the feasibility of wavelength division multiplexing technology is dependent on the development of high resolution, high throughput optical filters.
Adoption of universal standard transmission channels for fiber optic transmission promotes efficient application of wavelength division multiplexing. The International Telecommunication Union (ITU) has adopted a standard channel definition providing a 45 channel system over a wavelength range of 1520 nm to 1565 nm with a uniform channel spacing of 100 GHz (approximately 0.8 nm). The universal standard of telecommunication transmitting frequencies ensures intercompatibility of optical telecommunications systems and promotes realization of the full benefits of wavelength division multiplexing. As conventional thin film dielectric filters are not capable of efficiently and accurately separating the closely spaced transmission channels of the ITU frequency standard, an immediate need exists for more precise demultiplexing optical devices capable of high resolution, high throughout optical filtering.
Improvements in wavelength division multiplexing technology have focused on development of (1) optical devices capable of combining multiple optical signals corresponding to different transmission wavelengths or optical channels into a single fiber and (2) optical devices capable of separating multiplexed optical signals comprising of a plurality of data streams into discrete optical signals corresponding to selected transmission wavelengths or optical channels. In addition, these efforts have focused on developing optical signaling technology capable of supporting the use of more closely spaced transmission channels. One method of achieving these goals involves the development of optical interleavers suitable for multiplexing and demultiplexing optical signals. Interleavers provide multiplexer functionality by combining two or more streams of optical signals into a single, plural optical signal stream and provide demultiplexer functionality by separating a plural optical signal stream into individual optical signal stream components, typically corresponding to odd and even transmission channels. Four primary types of interleaver devices have emerged over the last several years: (1) interferometric optical interleavers, (2) dielectric thin film and birefringent filters, (3) planar wave guides and (4) fiber-based devices. Interferometric optical interleavers are especially promising for wavelength division multiplexing applications because of their low cost, wide free spectral range and fiber compatibility.
Interferometric optical interleavers are devices that replace at least one of the reflecting mirrors of a dual beam interferometer with a Gires-Toumois etalon (GT etalon). Over the last several years, interferometric optical interleavers have proven very useful in a variety of multiplexing and demultiplexing applications. Interferometric optical interleavers and deinterleavers operate by multiple-beam optical interference generated by the separation of an incident light beam into two sub-components that separately undergo phase modification, are coherently recombined and undergo constructive or destructive optical interference. FIG. 1 illustrates an interferometric optical interleaver (10) of the prior art comprising a cube-type beam splitter (15) in optical communication with a GT etalon (20) and an air gap phase control element (25), which are positioned along orthogonal axes with respect to each other. During operation, an incident beam (27) is directed onto the beam splitter (15), which separates the incident beam into first beam component (30) and second beam components (35), propagating on axes that are orthogonal to one another. The first component is directed through an air gap phase control element (25) and is reflected back toward the cube-type beam splitter by an external reflector. The second component is directed onto a GT etalon (20) wherein it is further separated into a plurality of sub-beams by a partially reflective internal reflector and a highly reflective external reflector. First and second beam components are coherently combined at the cube-type beam splitter and undergo optical interference. The nature and extent of the optical interference experienced depends on the optical paths of each beam component and the reflectivities of the reflectors comprising the GT etalon and air gap phase control element. As a result of optical interference, only certain frequencies of light are transmitted through the interleaver as output beams (40) corresponding to the transmission bands of the optical filter. By selection of the appropriate optical path length difference for reflected and transmitted beam components, interferometric interleavers of the prior art are capable of providing transmission spectra comprising periodic, substantially square-wave transmission bands.
U.S. Pat. No. 6,304,689 discloses a multifunctional optical filter capable of functioning as an optical interleaver. The optical filter described comprises a Michelson interferometer or Tynman-Green interferometer having a GT etalon substituted for one of the reflecting mirrors. Specifically, the patent discloses an orthogonal interferometer geometry employing a cube-type central beam splitter in optical communication with a GT etalon and reflective surface positioned along axes that are perpendicular to each other. The disclosed orthogonal optical filter design is reported to provide a substantially, square-wave transmission spectrum with selectable free spectral range, useful for minimizing cross talk during signal demultiplexing.
Although the optical filters in U.S. Pat. No. 6,304,689 are reported to provide spectral characteristics beneficial for multiplexing and demultiplexing applications, the disclosed design is susceptible to substantial problems arising from structural limitations inherent to fabrication of devices employing an orthogonal interferometer geometry. Conventional methods of fabricating optical devices, including optical arrangements having an orthogonal orientation, are extensively described in Moore et al. in xe2x80x9cBuilding Scientific Apparatusxe2x80x9d, Addisonxe2x80x94Wesley Publishing Co, 1989, pgs 119-256 and Fabrication Methods for Precision Optics, Hank H. Karow, John Wiley and Sons, 1993, pgs. 35-51, 442-462, 560-563, 644-671 and 714-721. First, the orthogonal arrangement of the prior art is susceptible to optical path length mismatch between the two legs of the Michelson or Tyman-Green interferometer. Optical path length mismatch is a deviation from the selected optical path lengths through each leg of the interferometer, which is introduced by fabrication errors. Mismatch errors result in substantial aberrations in the frequency, bandwidth and shape of the transmission bands because it is the selected difference in the optical path lengths of first and second beam components that establishes the net optical interference resulting upon beam recombination. Specifically, optical path length mismatch leads to substantial deviations from the desired square-wave shaped transmission bands of optical interleavers, which reduces light throughput and increases cross talk. Minimizing optical path length variation, therefore, is a vital factor in achieving a manufacturable optical interleaver design that is capable of providing square-wave transmission bands at precise, selected frequencies. Indeed, the piece-to-piece optical path length variation observed in optical filters having an orthogonal interferometer arrangement necessitates the use of optical path length compensation schemes, which add considerably to the complexity and cost of prior art interleavers.
Second, the optical configuration disclosed in U.S. Pat. No. 6,304,689 is also vulnerable to angular mismatch in the combination of the first and second beam components corresponding to each leg of the interferometer. This sensitivity arises from deviations in the relative positions of the cube-type beam splitter and each orthogonal leg of the interferometer, namely pyramidal error. In addition, susceptibility to angular mismatch derives from difficulties in fabricating a beam splitter in which the partially reflective surface is positioned at an angle precisely 45 degrees from the internal ends of each orthogonal leg of the interferometer. Angular mismatch causes poor vertical and horizontal beam recombination, which results in a substantial degradation in interleaver performance. Specifically, angular mismatch leads to high insertion loss, which decreases transmission of light having frequencies corresponding to the transmission bands and increases unwanted transmission of light having frequencies outside the transmission bands. High insertion loss, therefore, results in poor light throughput and increased cross talk.
U.S. Pat. No. 6,252,716 discloses a bulk optical interleaver comprising a Michelson interferometer in which both reflecting mirrors are replaced by GT etalons. Although the dual GT etalon interleaver design is reported to provide improved transmission and channel stability with respect to temperature variations, the design employs an orthogonal interferometer geometry highly susceptible optical path length mismatch and poor angular recombination.
U.S. Pat. No. 6,169,626 discloses an interleaver comprising a Michelson interferometer in which the first reflecting mirror is replaced by a GT etalon and the second reflecting mirror is replaced by a nonlinear phase control element. The disclosed interleaver design is reported to provide a means of upgrading a broader channel scheme into a narrower one. The optical arrangements described, however, are limited to orthogonal interferometer geometries, which are susceptible to optical path length mismatch and poor angular recombination.
U.S. Pat. Nos. 6,725,322 and 6,386,718 provides methods and device components for adjusting the optical path lengths of first and second beam components in interferometric optical interleavers having an orthogonal interferometer geometry. U.S. Pat. No. 6,725,322 describes the use of a tilt plate located in an air gap cavity for varying the optical path length through the air gap cavity. U.S. Pat. No. 6,386,718 provides air gap cavities having selectably adjustable cavity gas pressure and composition, which are capable of varying the effective optical thickness. While the methods and device components described in U.S. Pat. Nos. 6,725,322 and 6,386,718 are reported to provide methods of compensating for fabrication related optical path length variations, the designs add substantially to the complexity, cost and difficulty of fabrication of interferometric optical interleavers.
It will be appreciated from the foregoing that a need exists for high throughput optical interleavers capable of substantially minimizing cross talk. Particularly, interleavers having substantially square-wave transmission bands, which do not exhibit high insertion loss are needed. Accordingly, it is an object of the present invention to provide methods, devices and device components capable of efficiently combining or separating closely spaced optical channels with reduced cross talk. The present invention provides improved high throughput, low cost optical interleavers with selectably adjustable free spectral range. In addition, optical interleaver designs are presented that drastically improve the ease of fabrication and achieve greater piece-to-piece reproducibility. Further, the present invention provides optical interference filters that provide improved light throughput and decreased cross talk without the need of costly optical path length compensation schemes.
This invention provides methods, devices, and device components for improving frequency discrimination and optical signal processing using optical interference filters and interferometers. In particular, the invention relates to methods, devices and device components for separating closely spaced optical channels with minimized cross talk. The invention provides optical interference filters that exhibit efficient transmission of light of selected optical channels with decreased light loss, particularly decreased insertion loss. The present invention includes tunable and fixed frequency optical interference filters. In addition, optical interferometers and optical interference filters are provided with minimized vertical and horizontal recombination errors and improved optical path length matching. More specifically, this invention provides optical interference filters and interferometers having spatially parallel phase control elements that are easily fabricated with high piece-to-piece reproducibility. Also provided are methods of fabricating optical interference filters and interferometers having parallel phase control elements.
In a preferred embodiment, the present invention comprises an optical interference filter having substantially parallel phase control elements capable of functioning as an optical interleaver. Specifically, optical interference filters and interleavers of the present invention are capable of (1) combining separate optical channel streams into a single, more dense, plural channel optical stream, (2) separating a plural channel optical stream into a plurality of less dense component optical streams, and (3) functioning as a channel dropping filter, channel passing filter or a band pass filter. Optical interference filters and interleavers of the present invention are capable of providing substantially square-wave transmission spectra with selectably adjustable free spectral range and resonance frequencies. In a preferred embodiment, an optical interleaver of the present invention is capable of providing periodic, substantially square-wave transmission bands over the wavelength range of about 1520 nm and 1630 nm. The square-wave transmission spectra of the optical interference filters of the present invention are particularly well suited for multiplexing or demultiplexing optical data streams corresponding to the transmission channels of a selected frequency standard, such as the ITU frequency standard.
An exemplary optical interference filter comprises a beam splitter for separating an incident beam into a first beam component and a second beam component, a first phase control element in optical communication with the beam splitter for receiving the first beam component, and a second phase control element in optical communication with the beam splitter for receiving the second beam component. The first phase control element has a transparent internal end positioned a selected optical path length from the beam splitter and further comprises a first external reflector. The second phase control element has a transparent internal end, positioned a selected optical path length from the beam splitter, and further comprises a second external reflector. In a preferred embodiment, the optical interference filter has a parallel interferometer geometry wherein the internal end of the first phase control element and the internal end of the second phase control element are located in substantially parallel planes with respect to each other. In an alternative preferred embodiment, the internal end of the first phase control element and the internal end of the second phase control element are located in substantially the same plane.
First and second external reflectors of the present invention are at least partially reflective and, preferably, are highly reflective having a reflectivity selected from the range of about 80% to about 100%. In a preferred embodiment, first and second external reflectors reflect substantially all of the first and second beam components, respectively. Highly reflective external reflectors are beneficial because they provide for substantially complete optical interference of first and second beam components. Substantially complete optical interference results in optical filters capable of efficient transmission of light with frequencies corresponding to a selected transmission channel or series of transmission channels. Further, substantially complete optical interference results in optical interference filters capable of preventing substantially all transmission of light having frequencies outside of a selected transmission channel or series of transmission channels, thereby, minimizing cross talk.
During operation, an incident optical beam is directed onto the beam splitter, which separates the incident beam into a first beam component and a second beam component. The first beam component is directed onto the first phase control element, undergoes a modification of its phase and is reflected by the first external reflector back to the beam splitter. The second beam component is directed onto the second phase control element, undergoes a modification of its phase and is reflected by the second external reflector back to the beam splitter. Phase modification can be achieved by first or second beam components propagating a selected optical path length or by separation of first or second beam components into a plurality of subcomponents which may undergo optical interference. The present invention includes other methods of phase modification well known in the art of optical interference. The first beam component and the second beam component are coherently combined at the beam splitter and undergo optical interference. The nature and extent of the optical interference varies with frequency and is dependent on the phase modification of first and second beam components by the first and second phase control elements. In a preferred embodiment, the beam splitter separates the incident beam into substantially equal first and second beam components, which undergo substantially complete reflection at first and second external reflectors, respectively. In an exemplary embodiment wherein the first phase control element is an air gap phase control element and second phase control element is a GT etalon, the first beam and second beam undergo constructive and deconstructive interference in a manner providing a transmission spectrum comprising periodic, substantially square-wave transmission bands with selectably adjustable free spectral range.
The parallel interferometer orientation of the present invention is beneficial because it allows for greater precision in the manufacture of optical interference filters and interferometers using conventional fabrication methods. First, the parallel interferometer design of the present invention provides an order of magnitude improvement in the observed piece-to-piece variation in the selected optical path length difference between the first and second beam components. For example, when manufactured using standard fabrication methods, optical interference filters of the present invention achieve a variation in the selected optical path length difference between the first and second beam components that is less than 0.3 xcexcm. This is a substantial improvement over the variation in the selected optical path length differences observed in optical interference filters employing an orthogonal phase control element geometry, which is greater than or equal to 5 xcexcm. The order of magnitude improvement in observed optical path length difference variation largely results from the ease of making optical surfaces that are substantially parallel, in contrast to making orthogonal optical assemblies. Minimizing piece-to-piece variation in selected optical path length difference provides optical interference filters with greater efficiency and lower insertion loss. Further, reducing piece-to-piece variation in selected optical path length difference to less than or equal to about 0.3 xcexcm and avoids the need for costly optical path length compensation schemes. Specifically, the high precision attainable in the selected path length difference of the present invention results in optical interference filters having an insertion loss significantly less than optical interleavers of the prior art.
Second, the parallel interferometer design of the present invention provides a substantial improvement in the extent of angular recombination of first and second beam components achievable over interleaver designs employing an orthogonal phase control element geometry. Horizontal angular recombination errors arise from a mismatch of the optical angles which establish the optical paths of first and second beam components. Interleavers having an orthogonal interferometer orientation achieve optimal angular recombination of first and second beam components by precisely matching a first angle (element 50 in FIG. 1) defined by the plane containing the beam splitter and the plane containing the internal end of the first phase control element and a second angle (element 55 in FIG. 1) defined by the plane containing the beam splitter and the plane containing the internal end of the second phase control element. To achieve optimal recombination of first and second beam components these angle must be precisely 45 degrees. Structural limitations in fabricating orthogonal interferometer optical arrangements, however, result in substantial deviations in these angles of greater than 20 arcseconds.
The parallel interferometer optical arrangement of the present invention converts the problem of matching optical angles defined by two orthogonal planes to a process involving matching two angles that share a common defining axis. Specifically, horizontal recombination of first and second beam components in the optical geometry of the present invention is optimized by matching a first optical angle defined by the planes containing the partially reflective coating and the phase control elements or path length compensation element and a second optical angle defined by the planes containing a reflective surface parallel to the partially reflective coating and the phase control elements or path length compensation element. Because these angles share a common plane, they can be matched very precisely to within about 1 arcsecond using conventional fabrication processes, preferably double side lapping techniques. Accordingly, the interleaver design of the present invention is desirable because it is capable of efficient recombination of first and second beam components and, therefore, reduced angular recombination errors. Minimizing angular recombination distortion is beneficial because it decreases insertion loss and provides an output beam that can be efficiently coupled to an optical fiber with little loss of light.
Finally, the parallel interferometer design of the present invention provides optical interleavers, which may be fabricated more easily and with lower cost. Specifically, the interleaver design of the present invention may be fabricated by processes that are substantially less complicated and involve fewer fabrication steps than methods employed for manufacturing interleavers having an orthogonal interferometer orientation. The parallel interferometer design of the present invention reduces the number of independent variables relating to the position of phase control elements involved in interleaver fabrication. In contrast to prior art interleaver designs having two independent position variables corresponding to the location of two different planes defining the internal ends of orthogonal phase control elements, the present interleaver design involves a single position variable corresponding to the plane defining both internal ends of the parallel phase control elements or path length compensation elements. Decreasing the number of independent variables relating to the position of the phase control elements reduces the observed variations in optical path length through first and second phase control elements and decreases the overall design complexity of the optical path length-matching scheme.
In a preferred embodiment, the first phase control element, second phase control element or both are air gap phase control elements capable of selectively modifying the phase of the first beam component, the second beam component or both. Preferred air gap phase control elements of the present invention further comprise a front plate in optical communication with the beam splitter and located a selected optical path length from the beam splitter. The front plate and first or second external reflector are separated by an air gap of selected optical path length and, optionally, selected index of refraction. Preferably front plate and first or second external reflectors are located in substantially parallel planes with respect to each other. In a more preferred embodiment, the front plate, external reflector and the internal end of the phase control element are each located in substantially parallel planes with respect to each other. In an alternate preferred embodiment, the internal end of the phase control element is the front plate. Preferred air gap optical path lengths for a given angle of incidence are selected from the range of about 100 nm to about 20 mm.
In an exemplary embodiment, an air gap alignment spacer or a kinematic mounting system is provided to maintain a substantially constant or selectably adjustable optical path length through the air gap for a given angle of incidence. Use of an alignment spacer is beneficial because it provides optical filters with substantially constant, fixed transmission bands or selectably, adjustable transmission bands. Alignment spacers may comprise a low thermal expansion material, such as a ultra low expansion (ULE) material, to achieve a substantially constant and stable optical path length over the temperature range of about xe2x88x9240xc2x0 C. to about 85xc2x0 C. Alternatively, alignment spacers of the present invention may comprise a piezoelectric element and/or electrooptic modulator operationally coupled to the front plate and external reflector of the air gap phase control element. In this embodiment, the optical path length through the air gap is selectably adjustable by controlling the voltage applied to the piezoelectric element and/or electrooptic modulator. Air gap phase control elements with selectably adjustable optical path lengths are beneficial because they provide tunable optical filters having transmission bands with selectably adjustable frequencies, which are capable of precise frequency matching to transmission channels of a selected frequency standard, such as the ITU frequency standard.
The use of air gap phase control elements in the present invention is desirable because it provides optical interference filters and interleavers that are especially thermally stable. Thermal stability provides for very stable transmission characteristics, namely resonance frequencies and a free spectral range, that do not vary significantly over the temperature range of about xe2x88x9240xc2x0 C. to about 85xc2x0 C. In addition, air gap phase control elements are beneficial because they provide an air gap having a selectably adjustable refractive index. Specifically, the refractive index of the air gap may be selected by varying the partial pressure, identity or both of one or more gases in the air gap. In this embodiment, the air gap is equipped with a gas inlet capable of maintaining a substantially constant pressure of gases in the air gap. Selective variation of the refractive index of the air gap provides optical interference filters that are tunable.
In another exemplary embodiment, the first phase control element, second phase control element or both are dielectric phase control elements capable of selectively modifying the phase of the first beam component, second beam component or both. Preferred dielectric phase control elements of the present invention comprise at least one dielectric material, having an internal end and an external end, in optical communication with the beam splitter and positioned a selected optical path length from the beam splitter. Dielectric materials of the present invention have selected optical path lengths for a given angle of incidence, preferably selected from the range of about 100 nm to about 20 mm. In a preferred embodiment, the internal end and the external reflector are located in substantially parallel planes with respect to each other. In an alternative preferred embodiment, the internal end of the dielectric layer is the internal end of the phase control element. Dielectric phase control elements are beneficial because they provide a substantially constant optical path length from beam splitter to external reflector for a given angle of incidence. Use of low expansion materials for the dielectric material is preferred to achieve a substantially constant optical path length over the temperature range of about xe2x88x9240xc2x0 C. to about 85xc2x0 C.
In a preferred embodiment of the present invention, the first phase control element, the second phase control element or both are etalon optical filters capable of separating a beam component into a plurality of sub-beams and manipulating the phase of each sub-beam. A preferred etalon optical filter of the present invention comprises at least a partially reflective internal reflector located in a plane substantially parallel to the internal end of the first phase control element, second phase control element or both. In a preferred exemplary embodiment, the internal end of first phase control element, second phase control element or both are the partially reflective internal reflector. The partially reflective internal reflector and the first or second external reflector are located in substantially parallel planes with respect to each other and thereby form a resonance cavity having a selected optical path length between them. Preferred resonance cavity optical path lengths range from about 100 nm to about 10 mm.
In a preferred embodiment, the first phase control element, second phase control element or both are GT etalons having a highly reflective external reflector and a partially reflective internal reflector. Use of a GT etalon as a phase control element in the present invention is preferred because it is capable of reflecting substantially all of the first beam component, second beam component or both. Therefore, optical interference filters employing GT etalon phase control elements are capable of providing substantially complete optical interference. Optical interference filters providing substantially complete optical interference efficiently transmit light having frequencies corresponding to a selected transmission channel or series of transmission channels and substantially prevent transmission of light having frequencies outside of a selected transmission channel or series of transmission channels. Further, optical interference filters of the present invention comprising a first GT etalon phase control element and a second GT etalon phase control element provide interleavers with highly square-wave shaped, periodic transmission bands and improved chromatic dispersion characteristics. Improved chromatic dispersion characteristics refer to interference filters that exhibit reduced broadening of optical signals with respect to time due to dispersion.
In an exemplary embodiment, etalon resonance cavities of the present invention are composed of any dielectric material. In an exemplary embodiment, the resonance cavity is a dielectric cavity layer of a selected optical thickness and path length: Alternatively, an optical interference filter of the present invention comprises an air gap resonance cavity of selected optical path length, wherein the space between the partially reflective internal reflector and second external reflector is occupied by a selected pressure of one or more gases or by a substantial vacuum. Selection of the pressure and identity of gases in the resonance cavity establishes the refractive index of the resonance cavity. In this embodiment, an air gap alignment spacer or kinematic mounting system is desirable to maintain a substantially constant or selectably adjustable optical path length through the resonance cavity for a given angle of incidence. In addition, an exemplary embodiment includes a gas inlet operationally coupled to the air gap for the introducing of one or more gases to the air gap and maintaining a substantially constant pressure in the air gap resonance cavity. Air gap resonance cavities of the present invention are capable of achieving a substantially constant cavity pressure. Air gap resonance cavities are beneficial because they provide optical interference filters that are thermally stable.
For a given angle of incidence, etalon resonance cavities of the present invention may have a substantially fixed, selected optical path length or may have a selectably, variable optical path length. Resonance cavities with a fixed optical path length are beneficial because they are capable of providing a very stable optical path length for a given angle of incidence, and, thus provide very reproducible transmission spectra. Alternatively, selective variation in optical path length may be provided by alignment spacers comprising a piezoelectric crystal or electro-optical modulator operationally coupled to the etalon reflector pair. Resonance cavities with a variable optical path length are beneficial because they are capable of providing tunable transmission characteristics. Specifically, interference filters of the present invention with a variable optical path length resonance cavity are capable of selectably adjusting resonance frequencies and free spectral range by variation of the optical path length, refractive index of the etalon resonance cavity or both. Tunable optical interference filters are desirable because they can be effectively frequency matched to selected optical channels of a given frequency standard, such as the ITU frequency standard.
Beam splitters of the present invention include beam splitters capable of separating an incident beam into two beam components and directing the beam components into parallel phase control elements. In an exemplary embodiment, the beam splitter of the present invention comprises a partially reflective, planar optical coating in optical communication with the first phase control element and a planar reflective surface in optical communication with the second phase control element. In a preferred embodiment, the beam splitter of the present invention has a parallel reflector geometry, wherein the partially reflective optical coating and reflective surface are located in substantially parallel planes with respect to each other. The partially reflective optical coating is capable of reflecting a first component of the incident optical beam into the phase control element and passing a second component of the incident optical beam to the reflective surface. Optionally, the beam splitter may further comprise an additional reflective surface located in a plane substantially parallel to the other reflective surface and positioned such that partially reflective optical coating is located between the two reflective surfaces. In this embodiment, the additional reflector steers the incident beam onto the optical coating for separation into first and second beam components.
In an exemplary embodiment, the parallel reflector geometry of the beam splitter is provided by a first prism element having a reflective surface, first beam coupling surface, first prism coupling surface and first phase control element interface and a second prism element having a reflective surface, second beam coupling surface, second prism coupling surface and second phase control element interface. First and second beam coupling surfaces may be wedged (not parallel) with respect to the first and second phase control element interfaces to minimize back reflections and spurious etalons. Further, first beam coupling surface may comprise an antireflective surface coating to achieve high light throughput into and out of the optical interference filter. First prism element and second prism are operationally coupled to provide efficient propagation of light through the beam splitter and the partially reflective optical coating is located at the optical interface between first and second prism elements. In a preferred optical arrangement, first prism element and second prism element are coupled in a manner providing a parallelogram beam splitter geometry. In this embodiment, the first phase control element interface and the second phase control element interface occupy substantially the same plane and first beam coupling surface and the second beam coupling surface occupy substantially the same plane. Thus, an overall parallelogram geometry is formed by parallel reflective surfaces of the first and second prism elements and parallel planes containing first and second beam coupling surfaces and the first and second phase control element interfaces. The parallelogram optical arrangement of the present invention provides a beam splitter in which the incident beam, first beam component and second beam component are parallel for all rotational orientations with respect to the incident beam and the reflective surface. The use of parallel incident and component beams are beneficial because it allows spatially precise incorporation of phase control elements having selected optical path lengths from beam splitter to external reflector. Further, the parallel reflector optical arrangement provides optical interference filters that are more easily frequency tuned by angle tuning methods well known in the art. Angle tuning refers to rotating the optical arrangement of the present invention with respect to the axis of propagation of the incident beam to achieve selective adjustment of the optical path length through the interference filter.
The parallel reflector geometry of the present invention has significant benefits related to the fabrication of interleavers of the present invention. First, parallel reflecting surfaces and optical coatings can be manufactured to high angular precision using well-developed planar-parallel fabrication technology, particularly double-sided lapping methodologies. The angular precision achieved by double-sided lapping improves the extent of both horizontal and vertical recombination of first and second beam components.
Second, the planar, parallelogram beam splitter itself comprises an interferometer having unequal optical path length legs, which is easily evaluated using conventional optical interterometry techniques. Specifically, the optical path length difference of first and second beam components can be easily measured by directing a tunable laser onto the first beam coupling surface and monitoring reflected light exiting the second beam coupling surface. This technique allows measurement of the optical path length difference to approximately 0.01 xcexcm. If a substantial deviation in the pre-selected optical path length difference is observed, the parallel reflective surfaces can be re-polished to achieve a difference in first and second beam component optical path lengths within the desired tolerances. Accordingly, the present interleaver design allows for fabrication methods providing iterative polishing and optical path length evaluation steps, in contrast to orthogonal interferometer designs of the prior art. The ability to precisely evaluate the difference in optical path length of first and second beam components iteratively, during fabrication results in less costly and more accurate methods of manufacturing optical interleavers.
Third, the planar, parallelogram beam splitter configuration of the present invention is easily coupled to parallel phase control elements, such as air gap phase control elements, dielectric phase control elements, etalons, GT etalons and multi-cavity interference filters. Coupling phase control elements to the parallelogram beam splitter configuration is greatly facilitated by having both phase control element interfaces occupy the same or plane or parallel planes. Preferred coupling methods include but are not limited to optical contact bonding and use of optical cements.
Finally, the parallel reflector geometry of the present invention provides methods of manufacturing optical interference filters that comprise substantially less fabrication steps than conventional methods of manufacturing optical interleavers having an orthogonal interferometer geometry. Double-side lapping techniques allow the fabrication of two parallel surfaces simultaneously, in contrast to methods of fabricating cube-type orthogonal beam splitters that require each surface to be individually worked. In addition, double-sided lapping techniques provide an improvement of approximately one order of magnitude in angular matching achievable over fabrication methods for conventional cube-type beam splitters having an orthogonal optical geometry.
In a preferred embodiment, the beam splitter comprises a 50/50-beam splitter capable of separating the beam into first and second beam components with substantially equivalent intensities. A 50/50-beam splitter is preferred because it is capable of generating two substantially equivalent beam components, which may be coherently combined to provide substantially complete optical interference. The beam splitter of the present invention may be polarization insensitive or polarization selective.
Reflective surfaces of the first and second prism elements are preferably configured to provide total internal reflection of the incident light beam and second beam component, respectively. Alternatively, the reflective surfaces of first and second prism elements may comprise thin film layers capable of providing high reflectivity. In a preferred exemplary embodiment, the reflective surface of the second prism element has a phase correcting surface coating, which minimizes the change in phase between s and p polarization states upon total internal reflection. Phase correcting surface coatings useable in the present invention include but are not limited to one or more thin films coatings comprising SiO2, Ta2O5, HfO2, MgF2, TiO2 and Al2O3. An exemplary phase corrective coating comprises a Ta2O5 layer having a thickness approximately equal to a half wave at 1550 nm. An alternate phase corrective coating comprises a 4 layer thin film sequence comprising alternating Ta2O5 and SiO2 layers having a thickness equal to a half wave at 1550 nm. Importantly, because the reflective surface is configured to provide total internal reflection, addition of phase connective surface coatings comprising thin films does not substantially affect the net reflectivity of the reflective surface. Use of a phase correcting surface coating is important to preserve the function of the polarization diversity scheme and to improve the, extraction of counter propagating channels. Minimizing the change in phase between s and p polarization states improves the extent of optical interference observed upon recombination of first and second beam components and reduces polarization-dependent losses. Accordingly, interleavers of the present invention having phase correcting surfaces are capable of providing substantially square-wave shaped transmission bands with greater transmission of light corresponding to selected transmission channels.
In another preferred embodiment, the optical interference filter of the present invention further comprises one or more path length compensation elements positioned between the beam splitter and the first phase control element, second phase control element or both. Preferred path length compensation elements have an internal end and an external end and are positioned such that their internal ends are substantially parallel to the internal end of the first phase control element, second phase control element or both. In an exemplary preferred embodiment, the internal end of the path length compensation element is substantially parallel to the internal ends of the first phase control element, second phase control element or both. Path length compensation elements function to provide an additional optical path for first beam component, second beam component or both. In a preferred embodiment, inclusion of a path length compensation element provides additional optical path length to the first beam component, reflected by the partially reflective coating, equal to the difference between the optical path lengths of first and second beam component through the beam splitter.
Inclusion of a path length compensation element in the optical interference filter of the present invention provides selectable control over the optical path lengths of the first beam component, second beam component or both by providing additional media for the light waves to propagate through. Path length compensation elements may provide optical interference filters having identical optical path lengths of first and second beam components from beam splitter to external reflectors. Alternatively, in a preferred embodiment comprising an optical interleaver, path length compensation elements may provide optical interference filters wherein the optical path length of the first beam component from beam splitter to external reflector is different from the optical path length of the second beam component from beam splitter to external reflector. The ability to selectably, adjust the optical path length of first beam component, second beam component or both provides control over the sum of phases realized upon combination of first and second beam components, which establishes the nature and extent of optical interference achieved. Path length compensation elements of the present invention may be of any optical path length. Preferred optical path lengths of path length compensation elements are selected from the range of about 100 nm to about 200 mm.
In addition, path length compensation elements may be selected to provide optical interference filters wherein the optical path length of first and second beam components through a particular material, such as fused silica, are equal. Optical interference filters having path length compensation elements that provide equal optical path lengths through a particular material, such as fused silica, are especially desirable because they provide filters that are thermally stable. Specifically, such a configuration provides an interferometer structure in which fused silica regions undergo equivalent thermal expansion or contraction associated with a given change in temperature. Accordingly, the selected difference in the optical path lengths of first and second beam components does not change significantly with temperature, providing for optical interference filters having constant transmission characteristic over the temperature range of about xe2x88x9240xc2x0 C. to about 85xc2x0 C. In addition, matching the optical path lengths of first and second beam components through a particular material, such as fused silica, minimizes temporal distortion of optical signals resulting from chromatic dispersion.
In a preferred embodiment, an optical interleaver of the present invention is an optical interference filter having a first GT etalon phase control element and a second air gap phase control element oriented in parallel interferometer geometry. In this embodiment, the optical thickness of the resonance cavity, L, is selected to provide selected free spectral range and resonance frequencies of the optical interference filter. To provide an interleaver having substantially square-wave shaped transmission bands, the optical path length of the air gap is selected to equal a value of approximately L/2 and at least one path length compensation element is employed to provide equivalent optical path lengths from the beam splitter to the partially reflective internal reflector of the GT etalon phase control element and from the beam splitter to the front plate of the air gap phase control element. A preferred optical interleaver of the present invention having substantially square wave shaped transmission bands and a free spectral range of about 100 GHz comprises a GT etalon with a resonance cavity having an optical path length equal to about 3 mm and an air gap having an optical path length equal to about 1.5 mm, arranged in a parallel phase control element geometry.
Optical interference filters of the present invention having a 200 GHz free spectral range comprise preferred filters for adding or dropping even or odd channels of the ITU frequency standard. Other free spectral range values, such as 6.25 GHz, 12.5 GHz, 50 GHz, 100 GHz, 200 GHz, 400 GHz and 800 GHz are useful for separating a plural signal into discrete signal streams other than those corresponding to even and odd channels. Optical filters having a first GT etalon phase control element and a second air gap phase control element are capable of providing a transmission spectrum comprising periodic, substantially square-wave transmission bands with selected band width, particularly well suited for multiplexing and demultiplexing applications. Square-wave transmission bands are useful for demultiplexing plural optical signal streams with minimum cross talk because the position of the transmission bands and bandwidth may be selectably adjusted such that the bands only overlap the frequency range corresponding to selected channels. Accordingly, such optical configurations efficiently transmit light corresponding to selected channels and effectively prevent transmission of light having frequencies outside the spectral range of selected transmission channels. The bandwidth of transmission bands of the optical interference filter of the present invention may be selectably adjusted by selection of (1) the reflectivity of the partially reflective internal reflector of the GT etalon phase control element, (2) the optical path length of the air gap cavity, (3) the optical path length of the etalon resonance cavity and (4) any combinations of these. Selection of a highly reflective internal reflector results in narrower transmission bands and selection of an internal reflector with lower reflectivity results in broader transmission bands. In a preferred embodiment, the partially reflective internal reflector has a reflectivity of about 14%, which provides an optimal bandwidth for multiplexing and demultiplexing optical channels of the ITU frequency standard.
In another preferred embodiment, an optical interleaver of the of the present invention is an optical interference filter having a first GT etalon phase control element and a second GT etalon phase control element oriented in parallel interferometer geometry. In one embodiment, first and second GT etalon phase control elements have the same resonance cavity optical path lengths for a given angel of incidence. In an alternative embodiment, first and second GT etalon phase control elements have resonance cavities with different optical path lengths for a given angle of incidence. Use of a first GT etalon phase control element with a resonance cavity optical path length approximately the same value as the resonance cavity of the second GT etalon phase control element provides an optical interleaver with highly square-wave shaped transmission bands and improved chromatic dispersion characteristics. Improved chromatic dispersion characteristics refer to interference filters that exhibit reduced broadening of optical signals with respect to time due to optical dispersion. Optionally, dual GT etalon optical interference filters providing interleaver functionality also comprise one or more air gaps in optical communication with the first GT etalon phase control element, second GT etalon phase control element or both.
Optical interference filters of the present invention may be used for frequency discrimination applications, such as wavelength division multiplexing and demultiplexing applications. In such applications, an optical interleaver of the present invention may be used in combination with conventional thin film filters to isolate a single optical stream from a multiplexed plural optical stream prior. To achieve frequency discrimination, a multiplexed signal comprising a plurality of closely spaced optical channels is first passed through an optical interleaver configured only to transmit selected transmission channels. The plural multiplexed optical signal transmitted by the optical interleaver comprises a plurality of substantially more broadly spaced channels (less dense), which may be efficiently separated by conventional dielectric optical filters, such as an etalon, cut off filter, multi-cavity thin film filter or any combinations of these optical components. Alternatively, series of optical interleavers of the present invention may be employed to achieve a greater degree of signal separation prior to isolation by convention dielectric optical filters and detection. For example, a greater degree of signal separation of optical channels corresponding to the ITU frequency grid can be achieved by optically coupling a first optical interleaver having a free spectral range of 100 GHz to a second optical interleaver having a free spectral range of 200 GHz. In this optical configuration, the first optical interleaver converts an incident plural optical stream into a second more broadly spaced (less dense) plural optical stream comprising only even or odd channel, which is transmitted to the second interleaver. The second interleaver only transmits even and odd channels of the second plural optical stream, thereby, creating an even more broadly spaced third optical stream, which may be efficiently isolated using conventional thin film dielectric optical filters.
The present invention includes methods of fabricating optical interference filters with substantially parallel phase control elements. A preferred method of fabricating optical interleavers having parallel interferometer geometry comprises the steps: a) simultaneously polishing two sides of a first prism element thereby forming a first prism coupling surface and a first reflective surface located in substantially parallel planes with respect to each other; b) simultaneously polishing two sides of a second prism element thereby forming a second prism coupling surface and a second reflective surface located in substantially parallel planes with respect to each other; c) depositing a thin film optical coating on the first prism coupling surface of the first prism element; d) coupling the first and second prism elements, wherein the coated first prism coupling surface of the first prism element is operationally coupled to the second prism coupling surface of the second prism element thereby creating a beam splitter having polished first and second reflective surfaces located in substantially parallel planes, first and second unpolished, phase control element interface surfaces located in substantially parallel planes and an un polished beam coupling surface; e) polishing the first and second phase control element interface surfaces of the beam splitter; f) polishing the beam coupling surface of the beam splitter; g) simultaneously polishing a two sides of a path length compensation element thereby forming an internal end and an external end located in substantially parallel planes with respect to each other; h) operationally coupling the internal end of the path length compensation element to the first phase control element interface of the beam splitter; i) polishing the internal end of a first phase control element and operationally coupling the polished internal end of the first phase control element to the external end of the path length compensation element; and j) polishing the internal end of a second phase control element and coupling the polished internal end of the second phase control element to the polished second phase control element interface surface of the beam splitter.